Ganymede is a moon, yet it is larger than the planet Mercury, may contain more water than exists on Earth's surface, and even generates its own magnetic field, making it less like a conventional moon than a full-sized world that happens to orbit Jupiter.

Ganymede is a moon, yet it is larger than the planet Mercury, may contain more water than exists on Earth's surface, and even generates its own magnetic field, making it less like a conventional moon than a full-sized world that happens to orbit Jupiter.

Calling Ganymede a moon is correct, but it does very little to prepare us for what the object actually is. It is the largest moon in the Solar System, with a diameter of about 5,262 kilometres. It has an iron-rich core, a rocky mantle, an immense layer of water and ice, an old and geologically complicated surface, a thin atmosphere, auroras and a magnetic field generated inside the moon itself. If it travelled around the Sun rather than Jupiter,…